1. Field of the Invention
The present invention relates to a distance measurement apparatus and, more particularly, to a distance measurement apparatus which performs active distance measurement without using an analog IC designed for distance measurement.
2. Description of the Related Art
Automatic camera operations, such as automatic exposure control and automatic film winding, have been developed. Recently, so-called "automatic focusing (AF)" has developed in which the lens of a camera is automatically moved to focus the image of an object on the film.
AF devices can be roughly classified into two types. The first is generally known as "passive type" which utilizes the data representing the luminance distribution of an object. The second is generally known as "active type" which projects a distance-measuring light beam to an object and measures the distance between the camera and the object from the light reflected from the object. The active AF device has a simple structure and comprises inexpensive components, and find its use in so-called compact cameras.
FIG. 1 is a block diagram of a conventional active AF device. As FIG. 1 shows, the active AF device comprises a sequence controller 2, an AF IC 4, a lens-driving unit 6, a driver 8, an infrared-emitting diode (IRED) 10, a light-projecting lens 12, a light-receiving lens 16, a light-receiving element 18.
The sequence controller 2 is designed to control the sequence of operations to be performed in the camera incorporating the AF device. More specifically, it receives the data which has been output by the AF IC 4 and which represents the distance l between the light-projecting lens 12 and an object 14, and controls the lens-driving unit 6 in accordance with the data output by the AF IC 4. Controlled by the sequence controller 2, the unit 6 operates the driver 8, which in turn energizes the IRED 10. The IRED 10, thus energized, emits an infrared beam, which is projected toward the object 14 through the light-projecting lens 12 and is reflected by the object 14. The reflected beam, or signal beam, is applied to the light-receiving element 18 through the light-receiving lens 16.
The longer the distance l, the smaller is the angle between the perpendicular to the surface of the element 18 and the path in which the signal beam travels to the element 18. Hence, the distance l can be determined from the position of the beam spot formed on the surface of the light-receiving element 18. The element 18 is a two-segment light-receiving element or an optical position sensing device (PSD). Whenever a signal beam is applied to the light-receiving element 18, forming a beam spot thereon, the element 18 outputs two signal currents I.sub.A and I.sub.B which vary in accordance with the position of the beam spot.
The signal currents I.sub.A and I.sub.B each consist of two components. The first component corresponds to the signal beam applied from the object 14. The second component, generally known as "stationary-light component," corresponds to the light superposed on the signal beam (e.g., the sunlight or part of the illumination light applied to the object 14). Unless the stationary-light component is removed from each signal current, the distance l can not be accurately calculated from the signal currents I.sub.A and I.sub.B.
The AF IC 4 comprises stationary-light removing circuits 20a and 20b for removing the stationary-light components from the signal currents I.sub.A and I.sub.B, respectively. The circuits 20a and 20b are preamplifiers which amplify only the photocurrents which the element 18 has generated from the signal beam. The AF IC 4 further comprises preamplifiers 22a and 22b, logarithmic compression circuits 24a and 24b, and a differential operating circuit 26. The preamplifiers 22a and 22b amplify the photocurrents which the element 18 has generated from the light superposed on the signal beam. The logarithmic compression circuits 24a and 24b perform logarithmic compression on the outputs of the preamplifiers 22a and 22b, respectively. The differential operating circuit 26 finds the difference between the outputs of the logarithmic compression circuits 24a and 24b.
Logarithmic compression and differential operation, both known in the art, are performed on the signal currents I.sub.A and I.sub.B in the AF IC 4. The ratio of the signal current I.sub.A to the signal current I.sub.B is thereby supplied from the AF IC 4 to the sequence controller 2. In accordance with the current ratio, I.sub.A /I.sub.B, the controller 2 controls the lens-driving unit 6, thereby focusing the image of the object 14 on the film.
It is the recent trend that the sequence controller incorporated in an active AF device is a central processing unit (CPU) which comprises a one-chip microcomputer. This is because CPUs have become inexpensive enough to be built as active AF devices designed for use in low-price cameras.
A CPU has many input-output ports and performs various functions. It can control external components, detect the operating conditions thereof, and perform arithmetic operations on the data representing the conditions detected--in accordance with the programs built in it.